The present invention generally relates to the field of variable capacitors. More particularly, the present invention relates to a novel liquid filled variable capacitor that operates at high frequency and high RF power.
Variable capacitors are used in a variety of different capacities and come in a number of different forms. An area of particular importance, in terms of the utility of variable capacitors, is the field of semiconductor RF fabrication apparatus in which an RF field is provided to establish a plasma with which various fabrication processes can be carried out. In such apparatus, RF power is supplied from a source to an electrode that is in communication with a plasma region within a chamber. Variable capacitors are used in RF power match networks to match the impedance of the electrode and the plasma, constituting an electrical load, to the impedance of a source which delivers RF power to the plasma. The purpose of a match network is to increase the energy transfer efficiency between the load and the source. If the impedance match is sufficiently accurate, a measurement of the capacitor value could provide an accurate measure for the RF load. There are several different plasma procedures to be considered in the general application of wafer processing: plasma etching, plasma deposition, plasma photo-resist stripping, ion sources implantation, plasma chamber cleaning, etc. The plasma for each of these procedures will have a different RF load associated with it.
The current trend in the plasma equipment industry is toward higher frequencies and higher RF powers to sustain the plasma. Many common RF designs use standard RF components that force the match network to be placed at large distances away from the electrode. At higher frequencies, however, these larger distances lead to power losses along the non-matched portion of the transmission line between the match network and the electrodes. The use of smaller RF components can shorten this distance, thereby reducing the power loss.
One of the RF components used in the match network is an RF power capacitor. The most commonly used RF power capacitors are vacuum variable capacitors, which have one set of movable concentric tubes forming a first plate and one set of fixed concentric tubes forming a second plate. The movable tubes are connected to a bellows. The movement of the bellows brings the movable concentric tubes in and out interdigitally between the fixed concentric tubes.
The capacitance of a capacitor is generally determined by its ability to store energy based upon the amount of charge accumulated on overlapped surfaces. Thus, the larger the capacitor, the greater the amount of stored charge, generally. This can be more easily seen from the equation:
Q=C*Vxe2x80x83xe2x80x83(1) 
Where Q is the total charge stored in the capacitor, C is the capacitance and V is the voltage between the opposite plates. Thus, the capacitance C of the device is determined largely by the geometry of the opposing plates. For a parallel plate capacitor, the capacitance is given by the equation:
C=k*∈o*A/dxe2x80x83xe2x80x83(2) 
Where k is the relative dielectric constant of the medium, ∈o=8.854exe2x88x9212 Farads/Meter is the permittivity of free space, A is the surface area of the overlapped portions of the plates, and d is the distance between the plates. The capacitance of the vacuum variable capacitor can be calculated using equation (2), where A is the combination of all opposing surface areas between the moving and fixed tubes, d is the distance between the moving and fixed tubes. As the amount of overlapped area changes, the capacitance changes.
A significant problem associated with the vacuum variable capacitor, however, is its relatively large size, which requires that it be placed a large distance from the matching network. As stated above, large distances between the capacitor electrodes and the matching network lead to power losses. Another problem with the vacuum variable capacitor is its degradation over time, due to the wear and tear of the bellows from repeated flexing. Additionally, the inductance of the vacuum bellows changes with time. Yet another problem with the vacuum variable capacitor is that the inductance of the bellows is in series with the capacitance. This inductance causes the self-resonance point of the capacitor to occur at a lower frequency. Therefore, high frequency operations of this type vacuum variable capacitor are limited. Additionally, vacuum variable capacitors have a very large power loss at high frequencies and large amplitude RF power.
U.S. Pat. No. 5,162,972, issued Nov. 10, 1992, assigned to the United States Navy, entitled xe2x80x9cLiquid filled variable capacitorxe2x80x9d, describes a liquid filled variable capacitor or pulse forming line (PFL). The capacitor provides variable frequency, impedance, and pulse length without changing the capacitor or PFL (pulse forming line) hardware. The capacitor is constructed from two or more conducting surfaces and a dielectric fluid mixture separating the conducting surfaces. A fluid supply system furnishes the dielectric fluid mixture to the conducting surfaces and provides for varying of the dielectric constant of the fluid and thus the capacitor operating characteristics, by varying the mixture composition. The fluid supply system has a mixing tank connected to both a supply of high dielectric constant fluid and a supply of low dielectric constant fluid. The high dielectric constant fluid and low dielectric constant fluid are mixed to obtain a dielectric fluid having the desired dielectric constant. A pump conveys the dielectric fluid between the mixing tank and the conducting surfaces while a heat exchanger controls the temperature of the dielectric fluid.
U.S. Pat. No. 5,867,360, issued on Feb. 2, 1999, assigned to Murata Manufacturing Co., Ltd., Nagaokakyo, Japan and entitled xe2x80x9cVariable capacitorxe2x80x9d describes a variable capacitor having a stator with a stator electrode and a rotor with a rotor electrode. The rotor and stator are both housed in a recess section of a casing while allowing the recess section to be closed by a cover, enabling the rotor to be brought into stable close contact with the stator.
U.S. Pat. No. 3,996,503, issued on Dec. 7, 1976, assigned to Tokyo Incorporated, Tokyo, Japan and entitled xe2x80x9cVariable Capacitorxe2x80x9d describes a variable capacitor which includes a plurality of stator plates supported on spaced parallel rods. A plurality of rotor plates supported on a shaft are arranged so that each rotor plate is placed a predetermined distance from the surface of the adjacent stator plate. This is accomplished by spacer members disposed between the adjacent stator plates as well as between the adjacent rotor plates. Each spacer member is made of metal wire, with a circular cross-section, and is shaped in the form of a ring. Use of the metal wire having a predetermined dimension is much more convenient than a tube or sleeve. Consequently fixing the distance between the stator plates and between the rotor plates can be performed with high accuracy.
The present invention provides a capacitor and method for varying the capacitance of the capacitor in order to accurately match load and source impedances. The capacitor comprises a housing and a number of pairs of fixed first vanes positioned within the housing and forming a first plate of the capacitor. The capacitor also includes a number of pairs of second vanes forming a second plate of the capacitor and mounted to rotate interdigitally between the number of pairs of first fixed vanes. Finally, the capacitor includes means for circulating a dielectric fluid between the first and second pairs of vanes. It can also be a gas such as SF6 even though that gas has little heat capacity and must be flowed at high rates. SF6 gas also has a dielectric constant close to 1 so the capacitance per unit area for a fixed separation between capacitor plates, or vanes, is a factor of 3 less than flourinert.